Our work continues to focus on two approaches to hematopoietic differentiation. First, we demonstrated previously that expression of a c- myc or c-myb transgene reversibly blocks terminal differentiation of a mouse erythroleukemia (MEL) cell line. By constructing and expressing mutant c-myb transgenes in MEL cells, we have demonstrated that the DNA binding and transactivation domains are necessary and sufficient for the c-myb mediated block. So far, we are unable to determine whether these oncogenes block differentiation indirectly by promoting proliferation or directly by disrupting the differentiation pathway. Using this model, our long term goal is to understand the molecular mechanisms which are responsible for the apparent inability of most hematopoietic tumors to differentiate. Second, we have developed a novel method for subtractive cloning by incorporating polymerase chain reaction (PCR) technology into the preparation and analysis of subtractive cDNA libraries. We have used this novel methodology to identify genes that are expressed in most murine plasmacytomas but rarely in B lymphomas. We have identified two classes of genes having this property: 1) genes differentially expressed in plasmacytomas and normal plasma cells but not in B cells; and 2) genes differentially expressed in plasmacytomas but not normal plasma cells. Curiously, a number of genes in the former category are expressed in pre-B and plasma cells but not B cells, suggesting shared functional properties of the cells at either end of the B cell maturation pathway. Recently, we have modified our method for cell preparation and use of PCR-based subtractive probes to identify differentially expressed genes. We have used this modified approach to prepare a subtractive probe that recognizes genes expressed in two unrelated human myeloma cell lines but neither of two unrelated lymphoblastoid cell lines. With this probe, we have thus far identified 36 genes that demonstrate this pattern of expression. Our long term goal is to identify genes that not only mark but also determine the phenotypes of murine and human plasmacytomas and terminally differentiated normal plasma cells.